Method of removing alumina scum from a continuous-casting mold

ABSTRACT

A method of removing alumina scum from the mold of an aluminumkilled continuously cast steel by adding calcium carbide to the molten metal, reacting the calcium carbide with alumina in the molten metal to form a liquid calcium aluminate, and removing the calcium aluminate from the steel.

United States Patent [19] Snow [ Feb. 27, 1 973 [54] METHOD OF REMOVINGALUMINA SCUM FROM A CONTINUOUS- CASTING MOLD Roland B. Snow, MountLebanon Twp., Allegheny County, Pa.

Assignee: United States Steel Corporation Filed: Aug. 26, 1971 Appl.No.: 175,331

lnventor:

US. Cl. ..164/82, 164/73, 164/55 Int. Cl. ..B22d 11/00 Field of Search..164/73, 82, 55

[56] References Cited UNITED STATES PATENTS 3,052,936 9/1962 Hamilton..164/82 Primary Examiner-J. Spencer Overholser AssistantExaminer-Vernon K. Rising AttorneyRalph H. Dougherty [57] ABSTRACT 9Claims, No Drawings METHOD OF REMOVING ALUMINA SCUM FROM ACONTINUOUS-CASTING MOLD In the continuous casting of steel, molten steelis poured from a refractory-lined vessel or ladle into an intermediatevessel or tundish and then into an openended, water-cooled, continuouscasting mold blocked at its lower end by a starter bar. As molten steelsolidifies, the starter bar and attached casting are moved downwardly bymeans of pinch rolls. When aluminumkilled steel is being continuouslycast, nonmetallic particles accumulate atop the molten steel in themold. This accumulation of non-metallics on the surface of steel istermed scum. The scum usually consists of macroscopic alumina particleson liquid steel. Frozen steel on the top surface of molten metal, withor without entrapped nonmetallics, is known as skull. Often skimmingsfrom the mold surface are essentially steel with only a few macroscopicparticles of alumina. There are a few clusters of alumina within themetal in the skull but the original surface skull layer consists of finealumina particles in steel and this dispersion approaches a cermet inform. Fine oxides dispersed in metal are termed cermet.

As soon as the surface layer of steel in the mold becomes sufficientlysaturated with M particles to form the cermet layer, the surface(cermet) layer is stabilized, the viscosity is greatly increased, and,as heat is dissipated to the water-cooled copper mold above the surfaceof the liquid, the thin crust of metal freezes. Once frozen, thethickness of the cnist increases because heat continues to be dissipatedfrom the surface. It is at this stage that the scum, as well as themetal crust, may become entrapped in the skin of the slab to form abreakout or an internal defect in the steel.

One method of casting in which the formation of mold scum is controlledis teeming with a special submerged nozzle and, while casting, coveringthe steel with a low-melting mixture of lime, fluorides and bora tes tocombine with the alumina particles. How ever, some of this mixtureusually becomes entrapped beneath the skin of the slab and, when rolled,the product usually shows seams. This condition is also intolerable.

It is the principal object of my invention to provide a method ofcontinuously casting scum-free aluminumkilled steel.

It is another object to provide a method of fluidizing scum and removingfluidized scum from aluminumkilled, continuously cast steel.

It is also an object to reduce internal defects in aluminum-killed,continuously-cast steel castings.

It is a further object to provide a method of casting aluminumkilledsteel that on subsequent rolling of the casting will result in aseam-free rolled product.

According to my invention I prevent alumina particles from forming astable cermet layer in the mold by continuously feeding a small amountof calcium carbide to the molten steel in the tundish. I add fromoneeighth to 2 pounds of calcium carbide per ton of steel teemed intothe tundish. The addition is preferably made at the point where thestream enters the molten metal pool in the tundish. The calcium carbideis pulled under the surface and moved turbulently throughout the tundishwhere it reacts with alumina in the steel. Calcium oxidizes to formcalcium oxide, which unites with the alumina particles to form a calciumaluminate.

The heat of formation of both calcium oxide and calcium aluminate, aswell as the partial burning of the liberated carbon from the calciumcarbide to carbon monoxide, raises the temperature of the steel in thetundish as well as in the mold which helps prevent further skullformation. When calcium carbide is fed to the surface of steel in themold, the heat generated is sufficient to melt skull. Much of thecalcium aluminate formed floats to the surface of the metal in thetundish where it remains until the end of the cast. Some of the calciumaluminate is poured into the mold where it continuously floats to thefaces of the casting by the action of the metal in the mold. The calciumaluminate then moves downwardly with the casting and subsequently eitherpeels or spalls off the surface of the casting, because of thedifference in coefficient of thermal expansion between it and steel.

While other calcium-bearing alloys such as calcium silicon orcalcium-silicon-barium alloys might be considered as addition agents,the addition of silicon-bearing alloys raises the silicon content of thesteel, and removal of the excess silicon is impossible.Calcium-silicon-barium alloy contains some aluminum, thus the additionalaluminum must be removed along with the alumina in the scum. Thiscompounds the problem.

Calcium carbideraises the carbon content only a small amount as somecarbon is burned off. Carbon is a much more readily controlled elementthan silicon.

The calcium carbide can be added to the mold at a position adjacent tothat at which the metal stream enters the mold. It can alternatively beadded in the tundish above the tundish nozzle or the molten metal streamfeeding the mold. Additions at these positions will cause the calciumions to deoxidize the steel and to flux alumina particles that may be inthe steel at either position or which forms in the steel because of thealuminum content and the oxygen content of the steel.

My usual calcium carbide addition is from oneeighth to two-thirds poundsper ton of steel, but a larger addition is required when the steel ishighly oxidized or where the tundish stream is ragged or excessivelyturbulent. Then up to two pounds of calcium carbide per ton of steel maybe added. When more than one-fourth pound of calcium carbide per ton isto be added, it is desirable to have a lower than normal carbon contentin the ladle.

It can readily be seen from the foregoing that l have invented a methodof continuously casting scum-free aluminum-killed steel in which thescum is fluidized and removed from the casting, resulting in reducedinternal defects in the casting and a resultant seam-free rolledproduct.

I claim:

1. In the continuous casting of an aluminum-killed steel wherein moltensteel is poured from a vessel into an open-ended mold to form a moltenmetal pool therein and a partially solidified casting is continuouslyremoved therefrom, the combination therewith of a method of removingalumina scum from the mold comprising the steps of:

adding calcium carbide to the molten metal,

exothermically reacting said calcium carbide with alumina in the moltenmetal and with the scum on the surface of the molten metal pool to forma liquid calcium aluminate,

floating the calcium aluminate toward the walls of the mold,

causing the calcium aluminate to solidify on the casting faces,

moving the casting downwardly out of the mold,

and removing the solidified calcium aluminate from the faces of thecasting.

2. A method as defined in claim 1 in which said calcium carbide is addedto the molten metal stream feeding the mold.

3. A method as defined in claim 1 in which said calcium carbide is addedto the molten metal in said vessel.

4. A method as defined in claim 3 in which said vessel has a bottom-pournozzle and said calcium carbide is added to said vessel directly abovesaid nozzle.

5. A method as defined in claim 1 in which said calcium carbide is addedto the molten metal in the mold at a position adjacent that at which thestream impinges on the molten metal pool.

6. A method as defined in claim 1 in which said solidified calciumaluminate is removed from said casting faces by peeling.

7. A method as defined in claim 1 in which said solidified calciumaluminate is removed from said casting faces by spalling.

8. A method as defined in claim 1 in which said calcium carbide is addedin the amount of from one-eighth to 2 pounds per ton of steel.

9. A method as defined in claim 1 in which said calcium carbide is addedin the amount of from one-eighth to two-thirds pounds per ton of steel.

2. A method as defined in claim 1 in which said calcium carbide is addedto the molten metal stream feeding the mold.
 3. A method as defined inclaim 1 in which said calcium carbide is added to the molten metal insaid vessel.
 4. A method as defined in claim 3 in which said vessel hasa bottom-pour nozzle and said calcium carbide is added tO said vesseldirectly above said nozzle.
 5. A method as defined in claim 1 in whichsaid calcium carbide is added to the molten metal in the mold at aposition adjacent that at which the stream impinges on the molten metalpool.
 6. A method as defined in claim 1 in which said solidified calciumaluminate is removed from said casting faces by peeling.
 7. A method asdefined in claim 1 in which said solidified calcium aluminate is removedfrom said casting faces by spalling.
 8. A method as defined in claim 1in which said calcium carbide is added in the amount of from one-eighthto 2 pounds per ton of steel.
 9. A method as defined in claim 1 in whichsaid calcium carbide is added in the amount of from one-eighth totwo-thirds pounds per ton of steel.